Is there a way to throw an exception without adding the throws declaration?

Question

I have the following situation.

I have a Java Class that inherits from another base class and overrides a method. The base method does not throw exceptions and thus has no throws ... declaration.

Now my own method should be able to throw exception but I have the choices to either

• Swallow the exception or
• Add a throws declaration

Both a not satisfying because the first one would silently ignore the exception (ok I could perform some logging) and the second would generate compiler errors because of the different method headers.

public class ChildClass extends BaseClass {

        @Override 
        public void SomeMethod() {
            throw new Exception("Something went wrong");
        }
}

Answer 1

Yes there is, using typecast to Runtime exception and throws a runtime exception.

Create an Exception helper class like this.

public class ExceptionHelper {
    public static  <T> void throwException(Throwable t) throws Throwable {
        throw (Throwable) t;
    }
}

class ServiceClass {
    public void actualFlow() {
        try {
            //somethinf
        } catch (Exception e) {
            ExceptionHelper.throwException(e);
        }
    }
}

Answer 2

In Java 8, throwing a checked exception without declaring it can be done more easily due to type inference.

public class Main {
    public static void main(String[] args) {
      throwException(new Exception("exception"));
    }
    
    public static <T extends Throwable> void throwException(Throwable t) throws T {
        throw (T) t;
    }
}

Answer 3

If you use Project lombok and want to throw checked exceptions without the throws declaration, you can add @SneakyThrows to the method:

public void yourCaller(){
    yourMethod();
}
@SneakyThrows
public void yourMethod(){
    throw new Exception("Something went wrong");
}

This can throw checked exceptions without the caller needing to catch them.

Lombok provides an annotation processor that modifies the code at compile-time. With @SneakyThrows, it catches and re-throws the exception without the throws declaration.

As described in the description of @SneakyThrows, it transforms code into something like that:

public void yourMethod() {
    try {
        throw new Exception("Something went wrong");
    } catch (Exception t) {
        throw Lombok.sneakyThrow(t);
    }
}

From the sources of Lombok.sneakyThrow():

public static RuntimeException sneakyThrow(Throwable t) {
    if (t == null) throw new NullPointerException("t");
    return Lombok.<RuntimeException>sneakyThrow0(t);
}

@SuppressWarnings("unchecked")
private static <T extends Throwable> T sneakyThrow0(Throwable t) throws T {
    throw (T)t;
}

As you can see, it uses generics to trick Java into thinking that this would be an unchecked exception as shown in this answer.

Answer 4

You can throw unchecked exceptions without having to declare them if you really want to. Unchecked exceptions extend RuntimeException. Throwables that extend Error are also unchecked, but should only be used for completely un-handleable issues (such as invalid bytecode or out of memory).

As a specific case, Java 8 added UncheckedIOException for wrapping and rethrowing IOException.

Answer 5

Yes there is a why but it is not recommended at all you can use :

Java unsafe package

getUnsafe().throwException(new IOException());

This method throws checked exception, but your code not forced to catch or rethrow it. Just like runtime exception.

Answer 6

Here is a trick:

class Utils
{
    @SuppressWarnings("unchecked")
    private static <T extends Throwable> void throwException(Throwable exception, Object dummy) throws T
    {
        throw (T) exception;
    }

    public static void throwException(Throwable exception)
    {
        Utils.<RuntimeException>throwException(exception, null);
    }
}

public class Test
{
    public static void main(String[] args)
    {
        Utils.throwException(new Exception("This is an exception!"));
    }
}

Answer 7

Why don't you throw an unchecked exception? This doesn't have to be declared.

Two alternatives are

• wrap with a checked exception with an unchecked one.
• don't let the compiler know you are throwing a checked exception e.g. Thread.currentThread().stop(e);
• In Java 6, you can rethrow the exception if it is final and the compiler know which checked exceptions you might have caught.
• In Java 7, you can rethrow an exception if it is effectively final, i.e. you don't change it in code.

The later is more useful when you are throwing a check exception in you code and catching it in your calling code, but the layers inbetween don't know anything about the exception.

Answer 8

I just want do add an alternative answer, purely as an FYI:

Yes, there is a way to throw a checked exception without adding the throws declaration, by using the sun.misc.Unsafe class. This is described in the following blog post:

Throw a checked exception from a method without declaring it

Sample code:

public void someMethod() {
  //throw a checked exception without adding a "throws"
  getUnsafe().throwException(new IOException());
}

private Unsafe getUnsafe() {
  try {
    Field field = Unsafe.class.getDeclaredField("theUnsafe");
    field.setAccessible(true);
    return (Unsafe) field.get(null);
  } catch (Exception e) {
    throw new RuntimeException(e);
  }
}

However, this is not recommended. It is better to wrap in an unchecked exception as outlined in the some of the other answers.

Answer 9

You can use any exception derived from RuntimeException or RuntimeException itself

or

use a try-block for the exception throwing code and handle it there

Answer 10

Here's an example for intercepting checked exceptions and wrapping them in an unchecked exception:

public void someMethod() {
   try {
      doEvil();
   }
   catch (IOException e)
   {
       throw new RuntimeException(e);
   }
}

Answer 11

you can catch the exception with try- catch block in your method overridden. then you don't need to declare throws- statement.

Answer 12

A third option is to opt out of exception checking (just like the Standard API itself has to do sometimes) and wrap the checked exception in a RuntimeException:

throw new RuntimeException(originalException);

You may want to use a more specific subclass of RuntimeException.